1. Field of the Invention
This invention relates to a static Scherbius system which controls the speed of a wound-rotor induction motor, and more particularly to a static Scherbius system which prevents damage to a rectifier and an inverter and prevents also failure of commutation by the inverter due to an overvoltage appearing at the instant of recovery of the AC power supply voltage after sudden momentary service interruption.
2. Description of the Prior Art
As is well known, a static Scherbius system has the advantage that it can effectively control the speed of an induction motor with high operation efficiency, and because of this advantage, the Scherbius system is widely used for the speed control of a motor such as that driving a pump.
The inverter used in the static Scherbius system is a so-called separately excited inverter which makes commutation utilizing the AC power supply voltage, and such an inverter fails to commute current and is rendered inoperative or disabled when the AC power supply voltage is subject to an unusual drop due to, for example, sudden momentary service interruption. Thus, when the load driven under control of the static Scherbius system is a water distributing pump in a water purification plant, the service interruption gives rise to a serious situation which is suspension of water supply.
Further, at the instant of recovery of the AC power supply voltage from the unusually low level to the normal level, a transient voltage or overvoltage corresponding to the condition slip s=1 is induced in the secondary winding of the induction motor driving the pump. The rectifier and inverter will be destroyed by the transient voltage of overvoltage level, or a current of value exceeding the commutation ability of the inverter will be supplied to the inverter, and the inverter will fail to perform its function of commutation.
A German patent application laid-open No. 2018025 entitled "Untersynchrone Stromrichterkaskade" has been proposed in an effort to suppress such a transient voltage induced at the instant of recovery of an AC power supply voltage subjected to an unusual drop due to sudden momentary service interruption. According to this German patent, a thyristor and a resistor are connected in series between DC buses of a DC intermediate circuit connecting the rectifier to the inverter, and appearance of an overvoltage is detected to turn on the thyristor so as to suppress the overvoltage by the resistor.
Due to, however, the fact that the thyristor is connected between the DC buses of the DC intermediate circuit in the cited German patent application, an extinction circuit is inevitably required so as to turn off the thyristor as soon as the AC power supply is recovered. Such an extinction circuit includes an extinction-purpose power source (for example, a capacitor), an extinction-purpose thyristor and a firing circuit for firing the extinction-purpose thyristor. A static Ward-Leonard system including such as extinction circuit is expensive.
Further, in the cited German patent, the thyristor is turned on after detection of appearance of the overvoltage. Therefore, the proposed system has such an additional defect that the protecting function is not fully reliable since there are various response delays including delayed detection of the overvoltage and delayed firing of the thyristor.